Core-shell nanoparticles with a metal core can be used in various applications, such as energetic, pyrotechnics, joining, medical imaging, liquid hydrocarbon fuels, munitions and energy storage. However, the presence of an oxide layer on the surface of the core metal nanoparticle can significantly reduce the performance of the core-shell nanoparticles. For example, intermetallic reactions in aluminum/nickel (Al/Ni) composites can have a dramatic increase in the rate, as well as velocity of reactions, when the particle size is reduced to a few nanometers in diameter. However, the presence of a passive Al2O3 layer on the surface of aluminum prior to coating with nickel will reduce the efficacy of Al/Ni core-shell structures for energetic applications. Calculations show that the presence of about 20 wt % oxygen in the powder reduces the amount of aluminum available for the energetic reaction to as little as 55 wt %, along with a reduction in the kinetics. It is not uncommon for aluminum powders to have 20 wt % oxygen. Consequently, the negative impact of an Al2O3 layer outweighs the benefits of high enthalpy nanoparticles.
Il'in et al. (IL'IN, A. P., GROMOV, A. A., TIKHONOV, D. V., YABLUNOVSKII, G. V., AND IL'IN, M. A., “Properties of Ultrafine Aluminum Powder Stabilized by Aluminum Diboride”, Combustion, Explosion, and Shock Waves, 2002, v38, No. 1, p. 123-126) have demonstrated the feasibility of stabilizing ultrafine aluminum particles by forming AlB2 on the surface immediately after synthesis of the particles. They synthesized AlB2-coated ultrafine aluminum particles by the electric explosion of a boron coated aluminum conductor. The heat resistance of these particles increased by 30-40° C., compared to ultrafine aluminum particles coated with an oxide or hydroxide layer. Upon heating to 660° C., the degree of oxidation of AlB2-coated aluminum particles was 6-16% lower than that of oxide/hydroxide coated aluminum particles. Further, the heat of combustion for AlB2 coated particles was 2-4 kJ/g higher, compared to oxide/hydroxide coated particles. This may be due to the fact that AlB2 also releases energy during combustion and act as an active energetic material, which can promote the vaporization of aluminum and increase the combustion temperature. There is no conversion of aluminum oxide to aluminum boride in the above described method.
ALBx Synthesis
According to the phase diagram of aluminum-boron, AlB12 forms at 975° C., and it reacts with liquid aluminum to form AlB2. Further, AlB12 is a stable phase at room temperature if the boron content is >44.5 wt %. Conventionally, Al—B alloys are formed by the addition of KBF4 in liquid aluminum, where liquid aluminum reduces boron halide to AlB2 and AlB12.
Kirillova et al (KIRILLOVA, N. V., KHARLAMOV, A. I., AND LOICHENKO, S. V., “Synthesis of a High-Boron Aluminum Boride via Borothermic Reduction of Alumina”, Inorganic Materials, 2000, v36, No. 8, p. 776-782), have demonstrated the feasibility of synthesizing aluminum boride using alumina and boron as starting materials. According to Kirillova et al., the following reaction takes place when Al2O3 reacts with boron:Al2O3+(x+2)B→AlBx+AlO⬆+2BO⬆Initially, boron is oxidized by alumina to B2O3, followed by the formation and removal of a volatile metal oxide. Subsequently, the reaction intermediate 9Al2O3.2B2O3 is formed. Finally, 9Al2O3.2B2O3 is decomposed, and all the alumina reduced with boron is incorporated into the borides of various compositions.Passivation Techniques for Aluminum Nanoparticles
Aluminum nanoparticles have been synthesized using a solution-assisted laser ablation technique, with oleic acid coating. The particles were minimally aggregated with an oxygen to aluminum ratio of 0.094 to 0.159. Aluminum nanoparticles coated with transition metal oxides are observed to have less aluminum oxide compared to uncoated particles. In the case of surface passivated pristine aluminum nanoparticles using perfluoroalkyl carboxylic acid self-assembled monolayers, the active aluminum content was found to be 15.4%, which is lower than the active aluminum content of conventional nanoscale aluminum particles. All of the above discussed methods do not provide a passivating layer that increases the combustion of aluminum nanoparticles.
Solvothermal Synthesis of Nanoscale Materials
In solvothermal synthesis, the chemical reaction takes place in a closed system in the presence of solvents (aqueous and non-aqueous), under pressure (usually between 1 atm and 10,000 atm), and at moderate temperatures (usually between 100° C. and 1000° C.). Solvothermal synthesis is used to synthesize nanoparticles of metals, metal-based compunds such as oxides, borides and carbides. Gu et-al (GU, Y., QIAN, Y., CHEN, L. AND ZHOU, F. “A mild solvothermal route to nanocrystalline titanium diboride”, J. Alloys and Compounds, v352, 2003, v325-327) have synthesized nanocrystalline titanium diboride, using a solvothermal process. They started with amorphous boron powder, titanium tetrachloride (TiCl4) and sodium. Sodium reduced TiCl4 to titanium, which reacted with boron to form TiB2. The presence of TiB2 was confirmed using XRD and XPS. The particle size for as-synthesized TiB2 was 15-40 nm.